Various kinds of electrolytes are recently used for many electrochemical devices such as lithium secondary batteries, electrolytic condensers, electric double-layer capacitors, electrochromic display devices, and dye-sensitized solar cells that are currently studied in various ways for further usage, and the importance on electrolytes is increased day by day.
In particular, lithium secondary batteries are most highlighted as a battery with high energy density and long life cycle. Generally, a lithium secondary battery includes an anode made of carbon material or lithium metal alloy, a cathode made of lithium metal oxide, and an electrolyte made by dissolving lithium salt in an organic solvent. Structural stability and capacity of lithium metal oxides are determined according to intercalation and disintercalation reactions of lithium ions. The capacity of lithium metal oxides is increased as a charge potential is raised, but the lithium metal oxides become structurally unstable accordingly. Such unstable structure of the electrode results in generation of oxygen, which may cause overheating in a battery or reaction with the electrolyte, possibly resulting explosion of the battery.
In recent, organic solvents most frequently used for electrolyte of a lithium secondary battery include ethylene carbonate, propylene carbonate, dimethoxy ethane, γ-butyrolactone (GBL), N,N-dimethyl formamide, tetrahydrofurane and acetonitrile. These organic solvents generally have high volatility and high ignitability, so a lithium secondary battery adopting such organic solvents may exhibit a problem in its stability, particularly high temperature stability.
In order to solve this problem, there has been proposed a method of using an imidazolium-based or ammonium-based ionic liquid as an electrolyte of a lithium secondary battery. However, such an ionic liquid may be reduced at a higher voltage than lithium ions in an anode, or imidazolium or ammonium cations may be inserted into the anode together with lithium ion, which rather deteriorates the battery performance.
Meanwhile, Korean Patent Registration No. 10-751203 and Korean Laid-open Patent Publication No. 10-2007-85575 disclose eutectic mixtures of lithium salt and amide compound such as acetamide, urea, methylurea, caprolactam, valerolactam, trifluoroacetamide, carbamate and formamide, expressed as predetermined chemistry figures, as an electrolyte. Such eutectic mixtures exhibit high thermal and chemical stabilities as well as relatively wide electrochemical window, so they solve the problems such as evaporation or ignition of electrolyte caused by the usage of the existing organic solvents.
Accordingly, the development of various eutectic mixtures as electrolyte is accelerated. In particular, there is an increased demand on eutectic mixture electrolytes having better high temperature stability and a smaller lowest limit of an electrochemical window for the use in electrochemical devices requiring various electrochemical characteristics.